(a) Field of the Invention
This invention relates to improved phenol-formaldehyde resins, and to methods of their preparation. In particular it relates to methods of incorporating into a phenol-formaldehyde resin waste materials resulting from paper making procedures, and to the modified resin thus obtained.
(b) Description of the Prior Art
Partly as a result of cost and partly as a consequence of available supplies it has long been the practice to prepare phenol-formaldehyde resins from materials which are neither pure phenol nor pure formaldehyde. In both cases materials of a technical level of purity are commonly used. But of these two, the phenol component is by far the more expensive. As a consequence many efforts have been made to replace the phenol either completely, for instance as in urea-formaldehyde and melamine-formaldehyde resins, or at least in part with another substance and thus obtain either a different resin with new uses (as is the case with the use of urea or melamine) or a resin with approximately the same properties but which is cheaper. In this context it must be understood that "replacement" does not mean the incorporation of an inert, or relatively inert, diluent for the phenol. What is sought is a replacement substance which will take part in the chemistry of both the resin-making process and, where applicable, the resin curing process. These restrictions severely limit the number of possible substances that can be considered as potential replacement substances for phenol in phenol-formaldehyde resin systems.
All paper-making processes which employ a cellulosic material feed, irrespective of whether the overall process is directed toward first quality bond paper or low quality board for packaging, include as an early step a procedure whereby the cellulosic feed, such as wood, straw or bagasse is reduced to a fibrous mass. That pulp preparation step also produces as a byproduct an aqueous complex mixture of organic materials. The water-insoluble part of this mixture is generally removed by a suitable procedure. The remainder of the mixture, comprising a complex solution in water (which is to be taken as including both emulsions and colloidal suspensions), is generally treated to remove at least some of the water and then discarded. Disposal of this material presents a very difficult pollution problem. Typically, the water is removed to provide an aqueous material containing about 50% to 55% of organic substances: that is, to a solids content of about 50% to 55%, by weight.
Regardless of whether the pulping process used is the sulphite process, the kraft process, or a thermo-mechanical process, even though the detailed constitution of this byproduct changes its major component is a lignin-derived material, preferably as a lignin sulphonate. It is known that lignin materials of this type are reactive toward formaldehyde, and thus various attempts have been made to use these papermills byproducts in resins as a phenol replacement. In U.S. Pat. Nos. 3,185,654 (issued May 25, 1965 and assigned to West Virginia Pulp & Paper Company); 3,227,667 (issued Jan. 4, 1966 and assigned to The Borden Company); 3,597,375 (issued Aug. 3, 1971 and assigned to Georgia-Pacific Corp.); and 3,886,101 (issued May 27, 1975 and assigned to Georgia-Pacific Corp.) are described procedures whereby the lignin-containing portion of these materials may be used in phenol-formaldehyde resin systems intended for use as adhesives, for example in the manufacture of plywood and similar products.
All of these procedures, and the many others that have been described from time to time, suffer from at least one of the following disadvantages. The most common is that if a useful resin is to be obtained it is necessary to purify or isolate the lignin material from the remainder of the materials in the liquor used. The second disadvantage is that chemical modification of the lignin often is required: e.g. U.S. Pat. No. 3,886,101 describes polymerizing lignosulfonates with an aldehyde or epoxy crosslinking agent, and U.S. Pat. No. 3,658,638 describes pre-reacting the lignosulfonate with a phenol. The third disadvantage, more common in the earlier work, it that the lignin-based material is used merely as an inert extender, and is not expected or even wanted to take part in the resin chemistry: as a consequence at least some impairment of the resin properties must result from the dilution.
None of the described techniques have been found successful, insofar as none of these known processes appear to be worked on a commercial scale. But the cost of phenol is steadily rising, and the pressure on paper and board mill operators to eliminate environmental pollution is increasing rapidly. There is therefore considerable commercial stimulus toward both the replacement of phenol, and the elimination of these paper and board mill waste liquors. Indeed processes which require the isolation or purification of the lignin-material from paper and board mill wastes are selfdefeating to an extent, since in isolating the lignin-material from the waste there is no decrease in the overall amount of waste that has, somehow, to be discarded. The point is that processing one pound of black liquor from a kraft process plant to remove from it the lignin-based material present still leaves one pound of a different liquid residue for disposal.